Air and oil coolers are required in an aero engine to dissipate heat from air and/or oil systems of the engine such that hot fluid flowing through the cooler is cooled by a bypass air stream passing through a bypass section. The coolers are often located in the bypass section of a turbofan engine. A surface cooler which has relatively smaller dimensions in height may define some embedded channels which for carrying hot fluid to be cooled, such as described in Applicant's U.S. Pat. No. 7,377,100. In order to improve a cooler's performance, the contact area of the cooler with the bypass air stream may be increased by having cooler components such as fins projecting into the bypass duct. However, because air in the bypass duct can flow at speeds approaching 0.5 MACH, pressure losses due to the protruding fins or other components of the cooler are considerable. It is desirable to reduce the total pressure loss caused by the protruding fins or other components of the cooler disposed within the bypass duct and at the same time to maintain the cooler's heat transfer performance.
Therefore, there is a need to provide an improved cooler in a turbofan engine.